Irene: Experts say valley flooding likely to increase

Flood waters have just begun to recede along Lock St. in Rotterdam Junction. Homes and backyards in this area are still under water because the old Erie Canal passes behind them and is still holding waters. Murphy Towing of Amsterdam removes an SUV form one of the driveways on this street.

CAPITAL REGION  When John Garver visits the Schoharie Valley, residents often ask him why they keep having 100-year floods.

To Garver, a Union College professor of geology who organizes the annual Mohawk Watershed Symposium, the frequency with which he hears that question is an indication of change. In recent decades, people who live in the Mohawk River watershed have experienced increased flooding, and they can expect this activity to continue, Garver said.

The main issue is the Schoharie Creek, which is discharging into the Mohawk River at a much higher rate than in the past. In an item written for this year’s Mohawk Watershed Symposium, held in the spring, he wrote, “Hydrological data suggest more water is entering the watershed in a variable and complex way. How do we plan for this? How do we manage a watershed that is changing?”

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Hydrology is the scientific study of the properties and effects of water on the earth’s surface, in the soil and underlying rocks, and in the atmosphere.

Garver said that the Schoharie Creek’s higher rate of discharge can be attributed to an increase in storms such as hurricanes and nor’easters.

In a paper titled “Changes in the Hydrology of the Mohawk Watershed and Implications for Watershed Management,” Garver wrote, “The importance of these events is that they can result in locally very high precipitation in the headwaters of the Schoharie (up to 10 inches or more in a few recent events), and very little precipitation elsewhere in the basin. Thus we hypothesize that the most dramatic and significant change in the hydrology in the Mohawk watershed is related to Atlantic-tracking storms, which have had a significant effect on flooding in the southernmost part of the watershed.”

Getting wetter

“By all accounts, we have entered a wet phase in the history of the basin, by some metrics, the wettest in recorded history,” the paper says. “This wet phase is demonstrated by an increase in slope instability, bank erosion and sediment transport in the main channels and their tributaries.”

Data from the National Weather Service shows that so far the 2000s have been a much wetter century than the 1900s.

“The 1900s were pretty dry,” said Steve DiRienzo, the warning coordination meteorologist at the National Weather Service in Albany. He said that rainfall appears to have a 200-year cycle, where a century of relative wetness is followed by a century of relative dryness; the 1800s, he said, were also a pretty wet century. “It looks like we’re going into another wet century,” he said. The number of storms has increased in the past decade, as have the size of those events.

Garver said that the recent flooding was not a 500-year flood, as many believe.

He also said that hydrologists would prefer not to use the terms 100-year and 500-year flood, and instead focus on probability. What would be considered a 100-year flood should be thought of as having a 1 percent chance of occurring in any given year, while a 500-year flood should be thought of as having a 0.2 percent chance of occurring in a given year.

John Sheehan, executive director of the Adirondack Council, said that flooding has increased in the Adirondacks, which he attributed to climate change.

“What we used to think of as a 50-year flood may be getting closer to a five- or 10-year flood,” Sheehan said. “There’s a trend toward much more severe weather and more damaging single events. We’re seeing landslides take place in places we’ve not seen before.”

Though the Mohawk is the state’s second-largest river system, Garver refers to it as the forgotten river. It has never gotten as much attention as the Hudson River, but that as flooding in the river increases, people have started to pay more attention to it. In April, the U.S. Army Corps of Engineers announced that it would fund a study of flooding issues in the Mohawk River basin.

Biggest floods

Garver said that the flooding on the Schoharie Creek, one of the Mohawk’s tributaries, was historic. Of the top 10 floods that have occurred on the Schoharie Creek, all but one — a hurricane in 1955 — have occurred since 1980, Garver said.

The Schoharie Creek has undergone big changes in the past couple of decades, while the Mohawk’s other big tributary, West Canada Creek in the southern Adirondacks, has barely changed at all. And though long-term records indicate that the West Canada Creek basin supplies about 23 percent of the water to the Mohawk, while the Schoharie Creek supplies about 18 percent, this is changing. Since 1996, the annual contribution of the West Canada Creek has decreased, while the annual contribution of the Schoharie Creek has increased — to the point that in eight of the last 15 years the Schoharie Creek has supplied more water to the Mohawk River than West Canada Creek.

Garver said that in light of data showing that the Mohawk River and its tributaries are more likely to flood, flood mitigation strategies for the basin need to be re-evaluated.

“It is unlikely that we can engineer our way out of this situation without using cost-prohibitive strategies,” Garver wrote. “We have dams and locks in many places in the watershed, but none are used for flood mitigation. … We will not be able to prevent flooding in the Mohawk watershed and instead we have to be smart as to how we live with this changing natural hazard.” He said that FEMA has bought out homes and businesses located in predicted floodways and that “it is hard to say if they will do that again because they are nearly out of money after a very expensive year for natural disasters in the U.S.”

“The concern is that if the hydrology is changing, then you’ve got to expect more of these things,” Garver said. “Everything’s off the table. You’ve got to change how you plan.”